Electrospray ionization, a process in which a liquid is subjected to a strong electric field and is formed into a stream of small, charged droplets leading to sample ionization in the gas phase, is one of the most versatile ionization techniques for the investigation of macromolecules. The electrospray ionization process has recently found application as a means of producing ions for mass spectrometric analysis. (See, e.g., Smith et al., Anal. Chem. 62:885, 1990; Mann, Organ. Mass Spec. 25:575, 1990; Henion et al., U.S. Pat. No. 4,861,988; Allen et al., U.S. Pat. No. 5,015,845.) Today, electrospray ionization mass spectrometry (ESI-MS) is a widely used technique in biological, biochemical, pharmaceutical and medical research for studying complex biological samples containing species such as peptides, protein, carbohydrates or nucleic acids. The information on a sample that such an ESI-MS system can provide can be further enhanced by on-line combination with a microseparation technique, such as capillary electrophoresis, capillary electrochromatography or microcolumn liquid chromatography. Unfortunately, however, in many cases, the routine use of ESI-MS is still limited by relatively time consuming optimization of operational conditions and by insufficient sensitivity.
Recent advances in the design of electrospray devices have led to the improvement in efficiency of the electrospray sample ionization process and, at the same time, to a decrease in the total consumed sample amount. In a typical application, a sample in liquid solution and usually, but not always, in ionic form is fed through a metallic needle held at high potential and aimed at an entrance port of a mass spectrometer. Typical needles are 100-300 .mu.m i.d. at 2-8 kV. The electrospray process causes the sample solution to form small, charged droplets which contain sample ions and solvent. The charged droplets ultimately desolvate leaving sample ions, some of which then enter the mass spectrometer for analysis. The electrospray needle is typically placed 1 to 20 mm in front of the mass spectrometer entrance port or sampling orifice, which often is heated to assist in the desolvation process. The needle is usually at high potential and the mass spectrometer at or near ground, although the polarities can be reversed. Current trends are to use very small needle tips, metal coated drawn glass with internal diameters of 1-10 .mu.m or even in the submicrometer range. This helps to utilize the sample more effectively by transferring the generated ions more efficiently to the mass spectrometer. Typically, a metalized glass micropipette is used as both the sample reservoir and electrospray needle. Extremely low sample flow rates (low nL/min) are generated by the electrospray action.
While microelectrospray (sometimes also called nanoelectrospray) seems to work very well for infusion studies, its use for coupling of capillary electrophoresis (CE) to mass spectrometry is limited to very narrow separation capillaries with the electrospray interface operating in the sheathless mode. Coupling of a more standard (routinely used) larger bore (50-100 .mu.m i.d.) CE capillary to a microelectrospray may require the use of a liquid junction type of interface with a flow of a supporting liquid solution, such as 1% acetic acid in 50% methanol. (See, Lee et al., U.S. Pat. No. 4,994,165.) The use of a pump, necessary for pumping of the spray solution through the electrospray needle, generates significant pressure at the junction point and can result in reverse liquid flow in the CE capillary. Although a counterpressure can be applied at the injection end of the CE capillary, this approach is not very practical since significant experimentation must be performed to find the proper counterpressure, and any change in the system settings (e.g., blockage of the electrospray interface capillary, change of its length or change of the buffer viscosity) will require another time consuming optimization process.